1. Field of the Invention
The present invention relates to an X-ray apparatus of the type having an exposure unit by means of which a predetermined surface segment of an exposure plane can be exposed using X-ray radiation, and having a digital X-ray detector that can be moved in the exposure plane.
2. Description of the Prior Art
For years, most of the imaging examination methods used in medical technology have been based on X-ray images. In recent years, digital imaging techniques have increasingly become established, in place of conventional radiography based on photographic films. These digital techniques have the considerable advantage that the development of film, which is time-consuming, is not required. Rather, the image preparation takes place by means of electronic image processing. The image is therefore available immediately after the exposure. In addition, digital X-ray exposure techniques offer the advantage of improved image quality and possibilities for subsequent electronic image processing, as well as the possibility of a dynamic examination, i.e., the recording of moving X-ray images.
The digital X-ray exposure techniques that are conventionally used include systems known as image amplifier camera systems, based on television or CCD cameras, storage film or foil systems having an integrated or external readout unit, systems with optical coupling of a converter film to CCD cameras or a CMOS chip, selenium-based detectors with electrostatic readout, and solid-state detectors having active readout matrices with direct or indirect conversion of the X-ray radiation.
In particular, for the past several years solid-state detectors have been in development for digital X-ray imaging. These detectors are based on active readout matrices, made for example of amorphous silicon (a-Si), coated with an X-ray converter layer or scintillator layer, made of, for example, cesium iodide. The incident X-ray radiation is converted in the X-ray converter layer into visible light, which is converted into an electrical charge in photodiodes of the readout matrix, and stored in spatially-resolved fashion. Related technologies likewise use an active readout matrix made of amorphous silicon, but in combination with an X-ray converter (e.g. selenium) that converts the incident X-ray radiation directly into an electrical charge. This charge is then stored in spatially-resolved fashion on an electrode of the readout matrix. The stored charge is subsequently read out electronically via an active switching element, is converted into digital signals, and is sent to an electronic image processing system.
The basis of these solid-state detectors is conventionally semiconductor substrates (or semiconductor “plates”), as are produced in large quantities for consumer electronics applications, in particular for thin-film transistor display screens. Generally, however, such a semiconductor substrate manufactured for consumer electronics has only comparatively small dimensions (typically approximately 20×20 cm), which is often insufficient for medical applications. The manufacture of large-format semiconductor substrates, which would be suitable specifically for the sizes required in medical applications, is hardly possible at reasonable cost due to the relatively small piece counts involved. For X-ray detectors, it is therefore standard to cut a number of conventional semiconductor substrates and to glue them together to form a large substrate. This production method is still very complicated and therefore expensive.
German OS 38 25 703 discloses an X-ray apparatus having a strip-shaped digital detector element that is moved in linear fashion over the surface to be scanned during the exposure. After each shifting of the detector element, a partial image is recorded. The partial images are subsequently added together to form an overall image in an evaluation circuit.
Conventionally, in such a method the X-ray detector is moved in step-by-step fashion through the imaged surface (irradiated area). A disadvantage of this is that a considerable period of time is required for the detector to scan the entire imaged surface. In medical applications in particular, a short exposure time is often an essential criterion, especially since the patient under examination must generally hold himself or herself motionless for the duration of the exposure in order to avoid blurring of the image contours.